From Sega Retro
The Super Multi-play is built similarly to Sega's own Team Player adapter (the original version), but features a switch allowing it to play games developed for use with the 4 Way Play. Because these two peripherals work in different ways, the Super Multi-play actually as three leads which can be plugged into the Mega Drive. The left lead acts like the original Sega Team Player (which only required one controller port), while the two on the right act like the 4 Way Play, and must be inserted in both controller ports to play 4 Way Play games.
The peripheral can also allow up to five players to play at once, assuming another controller is inserted into the Mega Drive's first controller port. There are issues regarding some six-button Mega Drive controllers, in which the peripheral sometimes decides the controllers are standard three-button ones, but this mainly applies to third party peripherals.
Unlike the Team Player, the Super Multi-play does not allow you to manually select how many controllers are plugged in, and therefore always gives the signal all four controller ports are occupied even if they aren't. This doesn't usually affect gameplay, however.